Smart phones sharply increase data service traffic when bringing interests and convenience to life of people, and particularly in a densely-populated place such as a commercial centre and a residential area, simplex cell planning has been hard to match with the sharply increasing service traffic, thereby greatly influencing user experiences. For solving these problems, a HetNet subject is proposed in the 3rd Generation Partnership Project (3GPP), that is, a small cell is additionally deployed in an existing macro cell, thereby improving capacity of a network to meet a requirement of increasing packet data traffic.
However, introduction of a small cell also brings a series of new problems, and a problem about robustness of handover between a macro cell and a small cell is one of typical problems. In related art, handover is implemented according to the following process:
Step 1: a physical layer of UE reports an internal measurement result to a Radio Resource Control (RRC) layer;
Step 2: the RRC layer performs layer-3 filtering on the internal measurement result reported by the physical layer, compares the filtered value with a reference value to judge whether event A3 is allowed to be triggered or not, and starts a Time-to-Trigger (TTT) timer when the event A3 is triggered;
Step 3: when all layer-3 filtered values reported by the physical layer fulfils with a condition of triggering the event A3 during operation of the TTT timer, the UE sends a Measurement Report (MR) to an Evolved Node B (eNodeB) after timeout of the TTT timer;
Step 4: after receiving the MR, a source cell (serving cell of the UE) initiates a handover preparation with a handover target cell in the MR, and receives a Handover Command (HO CMD) sent by the target cell through an X2 interface;
Step 5: after the handover preparation is finished, the source cell sends the HO CMD to the UE; and
Step 6: after receiving the HO CMD sent by the source cell, the UE initiates a handover procedure to the handover target cell, namely initiates a random access process, and implements handover to the target cell.
In the handover procedure in the related art, long time (i.e. TTT+Time of Handover Preparation procedure, not including the time for transmitting and processing a signalling) is required from triggering of the event A3 to sending of the HO CMD. In a HetNet scenario, when UE moves from a macro cell to a small cell, signal strength of the small cell (i.e. a target cell) may rapidly increase because a cell radius of the small cell is relatively small. For UE still employing the macro cell (i.e. a source cell) as a serving cell, if waiting for a long time according to an existing protocol, the UE is likely to go deep into the small cell and may not correctly receive or receive an HO CMD sent by the source cell due to excessive interference of the small cell, thereby causing a handover failure, greatly influencing handover robustness of the UE and further influencing performance of a network. On the contrary, when UE moves from the small cell to the macro cell, the signal strength of the macro cell (i.e. the target cell) may rapidly increase because the cell radius of the small cell is relatively small. For the UE still employing the small cell (i.e. the source cell) as a serving cell, if waiting for a long time according to the existing protocol, the UE is likely to go deep into the macro cell and may not correctly receive or receive an HO CMD sent by the small cell due to excessive interference of the macro cell, thereby causing a handover failure and further influencing the performance of the network. The abovementioned conditions are more obvious when the UE moves faster.
In addition, the above handover procedure is described mainly with the event A3 as an example. In the existing protocol, besides the event A3, measurement events also include event A1, event A2, event A4, event A5, event A6, event B1 and event B2, and all of these events may also trigger handover procedure and may also face the problems in the abovementioned example.
Therefore, inter-cell handover in a related scenario has the problems of low handover success rate and poor handover robustness.